This invention relates in general to polyimide resins and, more specifically, to compositions and methods for making resilient, flame retardant, non-friable polyimide foams.
Prior U.S. Pat. Nos. 3,554,939 and 4,296,208 disclosed and claimed certain polyimide compositions which are flame resistant and useful as foams.
The polyimide compositions described in the abovementioned prior patents are made by first treating tetracarboxylic acid dianhydride with an esterifying solvent to produce a half ester.
The polyimide forming material is then prepared by adding to each mole of half ester, two moles of a mixture of suitable diamines at least 1.4 moles being an aromatic diamine, producing a viscous fluid, containing an intimate, unpolymerized solution which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution may be coated onto a surface and dried by heating to a temperature in the 150.degree.-190.degree. F. range for 14 to 18 hours. Thereafter the dry material may be crushed, pulverized and sieved. Alternatively, the composition may be spray dried. The powder then may be converted into a foam by various techniques such as dielectric, thermal and microwave heating.
The resulting foam is flexible, resilient and generally suitable for use in insulation and, to an extent in seat cushions. We have found, however, that this prior foam material has a limited life span in applications, such as seat cushions, where it is repeatedly compressed and released. When exposed to dynamic fatigue, these foams tend to particulate and eventually, totally disintegrate.
Thus, there is a continuing need for improved polyimide foam compositions which combine the flexibility, resiliency, flame resistance, and lack of toxic emissions when charing with the ability to resist vibration, shock and high cycle dynamic fatigue without particulating or otherwise disintegrating.